The present invention relates to liquid impermeable moisture vapour permeable composite structures comprising a specific thermoplastic film which is thinly coated onto a support layer. The composite structures of the present invention can find a variety of applications, wherein moisture vapour permeability is desirable for example within absorbent articles such as diapers, sanitary napkins, panty liners and incontinence products, protective bedding covers, protective clothing and the like.
Thermoplastic films which provide a liquid barrier in addition to providing moisture vapour permeability are known in the art. Particularly preferred are hydrophilic continuous films that do not allow the flow of moisture vapour through open pores or apertures in the material, but do transfer substantial amounts of moisture vapour through the film by absorbing water on one side of the film where the moisture vapour concentration is higher, and desorbing or evaporating it on the opposite side of the film where the moisture vapour concentration is lower. Compositions used to form hydrophilic continuous, moisture vapour permeable, liquid impervious films are also said to be moisture vapour permeable, liquid impervious.
For example WO 95/16746 discloses films prepared from mixtures of a) block copolyether ester, block copolyether amides (e.g. Pebax(trademark)) and or polyurethane and b) thermoplastic polymer which is incompatible with a, and c) a compatibiliser. The films are liquid impermeable and have moisture vapour permeability of about 700 g/m2/day. Also, U.S. Pat. No. 5,447,783 discloses a vapour permeable water resistant multi component film structure having at least three layers. The outer layers are hydrophobic copolyetherester elastomers having a thickness of 1.3-7.6 micrometers and a WVTR of 400-2500 g/m2/24 hrs. and the inner layer is a hydrophilic copolyetherester elastomer having a thickness of 7.6-152 micrometers and a WVTR of at least 3500 g/m2/24 hrs.
U.S. Pat. No. 5,445,875 discloses a waterproof, bloodproof and virusproof breathable laminate. The laminate comprises a woven/nonwoven fabric and an extruded film such as Hytrel(trademark) having a thickness of about 1 mil.
However, whilst these films are desirable in so far as their moisture vapour transport rate is concerned, these films tend to lack other desirable physical characteristics such as tear strength and flexibility depending on the end usage envisioned.
Attempts to address this problem have resided in providing the polymers on a supportive substrate which provides the required physical attributes. Such composites are described for example in U.S. Pat. No. 5,599,610 which discloses tri-laminated fabric for surgical gowns comprising outer layers of woven fabric and an inner layer of a microporous polyurethane membrane. The microporous film has a thickness of 12-55 micrometers and a MVTR of 1100 g/m2/24 hrs. upright and 5500 g/m2/24 hrs. inverted (ASTM E96-B). Polyether-polyurethane adhesive is used to join the layers.
Similarly, U.S. Pat. No. 5,532,053 discloses a high moisture transmission medical film which can be laminated onto a nonwoven material. The laminate film comprises a first layer of polyetherester copolymer and second and third layers selected from a specified group of polymers. The film has a MVTR of greater than 750 g/m2/24 hrs. (ASTM F1249) and a thickness of less than 1 mil (25.4 micrometer) preferably 0.6 mm to 0.75 mm (15-19 micrometers).
U.S. Pat. No. 4,938,752 discloses absorbent articles comprising films of copolyether esters which have reduced water permeability, a water vapour permeability of 500 g/m2/24 hrs. (as measured in a specified described test) and a thickness of 5-35 micrometers. There is no disclosure of a supportive substrate.
U.S. Pat. No. 4,493,870 discloses a flexible layered waterproof product comprising a textile material covered with a film of a copolyetherester having an MVTR of at least 1000 g/m2/24 hrs. (ASTM E96-66) having a thickness of 5 to 35 micrometers.
GB 2 024 100 discloses a flexible layered water resistant article comprising a microporous hydrophobic outer layer which is moisture vapour permeable but resist liquids and a hydrophilic inner layer of polyetherpolyurethane having a MVRR of above 1000/m2/24 hrs.
However a problem with the films disclosed in the prior art is that the thickness at which the films can be readily extruded onto a support substrate is severely limited using standard extrusion techniques. Furthermore such films often require the addition of an adhesive in order to ensure permanent fixation of the coating to the substrate. However, this in turn detrimentally affects the overall moisture vapour permeability of the resulting composite.
Hence, there is a need to provide a composite structure which is liquid impermeable and moisture vapour permeable and which is readily processable so as to provide a thin film coating.
The present invention relates to a moisture vapour permeable liquid impervious composite substrate.
Said substrate comprises:
(a) a thermoplastic film made form a moisture vapour permeable thermoplastic composition having a complex viscosity of from 50 poise to 4000 poise at a frequency of 1 rad/s, at a temperature of 210xc2x0 C. or less, and a complex viscosity of less than 2000 poise at a frequency of 1000 rad/s and at a temperature of 210xc2x0 C. or less, said film having a thickness of less than 10 micrometers to 0.5 micrometers and
(b) a support layer, said support layer being moisture vapour permeable.